1. Technical Field
The present invention relates to a digital cordless telephone system, and particularly to a method of accurately detecting and indicating an outer communication range in a digital cordless telephone system.
2. Background Art
In general, conventional cordless telephone systems available in the market today operate on ten or more separate frequency channels. These telephones have generally been used to allow the user to make and receive telephone calls with a cordless handset unit connected by a radio link to a fixed base station. Because of radiated power limitations on the radio link, however, the handset unit must be used within a communication range of a few hundred feet of the base station. The actual communication range of operation in any given situation depends on the radio propagation characteristics of a particular environment. It is extremely important that the communication range between the handset unit and the fixed base station be monitored and the user be notified when the handset unit is out of the communication range so that the user can return to the coverage area; otherwise, the communication line can be disconnected.
Conventionally, there are several known techniques for determining whether the handset unit is within the communication range of the fixed base station of a cordless telephone system. For example, in U.S. Pat. No. 4,939,766 for Radio Telephone System, Umemoto et al. disclose a cordless telephone system in which a predetermined threshold value is set for comparison with the intensity of a received electric field generated in the radio line so that, when the received electric field intensity does not reach the threshold value because the handset unit is outside of the communication range from the fixed base station, an alarm tone is generated to notify the user that he is out of the communication range from the fixed base station. One difficulty in implementing functional out-of-range indicators of Umemoto '766 is that conventional cordless telephone systems use a time division duplex (TTD) transmission scheme on a single RF channel. TTD transmission optimizes the use of the available radio frequency spectrum by allowing transmission of voice and data in both directions between the fixed base station and the cordless handset unit. Specifically, the cordless handset unit and the fixed base station alternately transmit and receive one-millisecond bursts of information every two milliseconds, one receiving while the other transmit. There is no reliable way for an unsynchronized receiver monitoring an active channel on which a call is in progress to tell whether it is monitoring a transmission from a handset unit or from a base station at any given moment in time. Moreover, it is very difficult to measure the received electric field intensity generated from the radio line accurately, and consequently the outer communication range on the basis of the electric field intensity. Therefore, most cordless telephone systems require separate circuitry constructed therein to accurately measure a received electric field intensity repeatedly for every time unit; otherwise, an alarm tone or a visual display of alarm message notifying the user that the handset unit is out of the communication range may occur unreliably.
Another method is disclosed, for example, in U.S. Pat. No. 5,193,216 for Detecting Out Of Range In Response To A Loss Of Signal And A History Of Approaching Out Of Range Prior To The Loss Of Signal issued to Davis, U.S. Pat. Nos. 5,203,013 and 5,327,578 for Radio Telephone System Supporting Busy And Out-Of-Range Indications issued to Breeden et al., and U.S. Pat. No. 5,373,548 for Out-Of-Range Warning System For Cordless Telephone issued to McCarthy, in which a RF signal level received from the handset unit is measured by the fixed base station to provide a warning tone at the earpiece of the cordless handset unit if the RF signal level is below a minimum threshold level. In Davis '216, if the RF signal is not detected, an "out-of-range" warning signal is generated to notify the user that the handset unit is out of its communication range based on a history of received signal strength values. In the cordless telephone system of McCarthy '548, the warning signal is provided such that even if the handset unit is beyond the communication range of the cordless telephone, the user can still hear the warning signal. The fixed base station will transmit the "out-of-range" warning signal to the handset unit for a predetermined number of times and then, if the cordless handset unit does not come back into the operating communication range of the base station, the base station will then disconnect from the telephone line. While these conventional methods of determining an outer communication range of a cordless telephone system have their own merits, I have found that further improvements can be contemplated.